On the software side, I've been using BackCountry Navigator for a couple years and love it. Though it does take a bit more computer savvy and set up time, it is absolutely great for offroad riding with the right prep. Unfortunately, that prep requires finding a source for Google Maps which is.... frowned upon, since it is stored on the device. But there are free OpenMaps alternatives.
I have been using an old Nexus1 as my offroad gps device for a while now, and thinking about upgrading to a 7 incher as well. It seems that the Kindle Fire HD is the best bet. They are said to have bright screens and they have been polarized for easier sunlight visibility. A Fire HD, Liquipel coating, Otterbox, and anti-glare screen protector sounds like just the ticket. My main issue is mounting. I mostly need it for singletrack, so it needs to be solid while reducing vibrations from the 2-stroke (not to mention roots and rocks). Any ideas?